There have been a number of ways that stability has been provided for in marine structures in the past.
Floating facilities either ship-shape or column stabilised, achieve stability through changing buoyancy by a part of their body moving in or out of water. It is because of this requirement that a large part of their body providing buoyancy and stability is exposed to maximum environmental loading. PA1 Submarines achieve static stability by ballast control using the power of pumps to pump water in or out of the vessel as required. PA1 Non-floating structures are stabilised either by piling to the sea bed as foundation or by being attached to large weights known as gravity foundations. PA1 There are also tethered structures which have tethers stabilised by gravity or piling, acting against the buoyancy of a floating vessel and keeping the tethers always in tension. The stability of the vessel is either provided by the tension in the tethers or a combination of the tension as well as the buoyancy changes due to the vessel coming in or out of the water. PA1 Dynamic positioning systems which use trusters to resist the wave forces. These are also used to turn a ship around to face the waves. Turrets are required to allow the ships to turn around a moonpool housing pipes connected to the source of the minerals. PA1 Tensioners can be used but these have the expense of the foundation as well as the tensioners. PA1 i) a first portion adapted to be connected to or form an integral part of the marine body, said portion incorporating a piston; PA1 ii) a second portion adapted to envelope the piston, the first and second portions being moveable with respect to each other, thereby creating a variable volume chamber; PA1 iii) sealing means adapted to form a substantially fluid tight seal between the piston and the second portion; PA1 iv) a constant pressure source adapted to maintain a constant pressure within the variable volume chamber; PA1 v) tether means adapted to tether the second portion to the sea bed; wherein the buoyancy assembly is adapted such that displacement of the marine body from its hydrostatic equilibrium position results in the generation of a restoring force, upwards or downwards, tending to restore equilibrium characterised in that that constant pressure source is provided by connecting the variable volume chamber to atmosphere. Atmospheric pressure represents a virtually infinite source of constant pressure and is immediately available if suitable connections are made. Preferably the connection to atmosphere is provided by pipework connections through the marine body supporting structure. In particularly preferred embodiment the piston is substantially circular in shape and the second portion takes the form of an inverted circular cylinder, closed at one end. PA1 a) providing one or more buoyancy assemblies as described; PA1 b) attaching the or each buoyancy assembly to the marine body; PA1 c) providing control means adapted to control the operation of the or each buoyancy assembly such that the marine body is stabilised at the desired point of hydrostatic equilibrium.
Providing piled stabilised foundation is expensive, requiring specialised crane vessels, pile driving hammers and the expense of the piles. Gravity stabilised foundation require large, usually concrete, structures and expensive ballast systems. These structures need to be either externally stabilised for transportation and installation offshore or their stability element would need to be water surface piercing attracting environmental loads. Structures sitting on the sea bed fully submerged would need crane vessels to lower them down or raise them up and still require to be stabilised by a foundation.
Floating facilities which are utilised as offshore platforms for mineral production require to keep station whilst being connected to the source of the minerals. However, as they need to be surface piercing for their function and stability, they are at times subjected to severe environmental loads. In order to minimise their motions and for station keeping a number of facilities have been developed. These include:
In summary, the known technology is expensive to install and expensive to operate on a day-to-day basis. No one technology can provide buoyancy and stability when partially and fully submerged as well as providing foundation stability to a structure sitting on the sea bed. No one existing marine stabilising system has the facility to alter its dynamic characteristics to suit changes in the environmental loads.
The closest prior art known to the applicant is the inventor's own earlier application PCT/GB95/02883. This describes a buoyancy assembly comprising the first unit and a second unit which, in combination contain a volume of compressed gas. Both the first and second units are free to move up and down in order to vary the volume of compressed gas contained therein.
Whilst this arrangement can maintain hydrostatic equilibrium it tends to require active rather than passive ballast control to be effective. This requires expensive equipment and complex control circuits.
It is the object of the present invention to overcome some or all of these disadvantages.
This invention relates to buoyancy assemblies that can provide hydrostatic stability even when fully submerged, and which have the facility for altering their dynamic characteristics and provide sea bed foundations that minimise or eliminate the vertical loads acting on the sea bed.